Monday marked 25 years since a car bomb exploded in an underground parking garage of the World Trade Center, killing six people and injuring hundreds.

The city held a solemn commemoration of the 1993 World Trade Center bombing, including a Mass at a church near the trade center and a ceremony on the 9/11 memorial plaza.

Afterwards, the World Trade Center's operating agency issued an apology, saying the agency and the country was wholly unprepared for an event that became a precursor to 9/11.

"We were not ready for what visited us that day," Port Authority of New York and New Jersey Chairman Kevin O'Toole said at the ground zero ceremony. "America was not ready for what visited them that day. And for that, I say: I'm sorry."

The ceremony also included a reading of victims' names and a moment of silence at 12:18 p.m., when the bomb exploded and became a harbinger of more terror at the twin towers eight years later.

The attention surrounding the 25th anniversary is "long overdue," said Judy Shirtz, sister-in-law of Stephen Knapp, who feels that the loss of families like hers has been overshadowed by 9/11 and its far greater toll.

"It happened to us first, it shouldn't have happened again, and it did," she said.

The bomb, in an underground parking garage, was set by Muslim extremists who sought to punish the U.S. for its Middle East policies, according to federal prosecutors.

"While overshadowed by 9/11, the 1993 bombing represented a pivotal moment in the history of the World Trade Center, in the history of New York City, and, frankly, our own national reckoning with terrorism in a global age," said Sept. 11 museum president Alice Greenwald, whose institution has a permanent exhibition on the bombing and a special installation to commemorate the anniversary. "It had so many of the elements that we would later come to associate with 9/11."

Six bombing suspects were convicted and are in prison, including accused ringleader Ramzi Yousef - a nephew of self-proclaimed 9/11 mastermind Khalid Sheikh Mohammed. A seventh suspect in the bombing remains at large.

An estimated 50,000 people fled the blacked-out twin towers, some groping their way down smoky stairs, others rescued from stalled elevators or plucked from rooftops by police helicopters. More than 1,000 were injured.

A memorial fountain dedicated to the 1993 bombing was crushed in the attacks that destroyed the towers on Sept. 11, 2001. But bombing victims' names are now inscribed on one of the memorial pools that bear the names of the nearly 3,000 people killed on 9/11.